Jellyfish have no brain - can they feel pain?

[Angrboda]

Or pleasure or...well, anything really? 

One for the ghost of Descartes probably but I'd say no - they feel nothing. Thoughts?

This is kind of tricky...

On the one hand, they must feel things in the sense that they react to their environment. 

On the other, they probably don't have concepts. So when they react to their environment, they just react, and there's no part of them conceptualizing "Geez, I'm scared; gotta get out of here." 

I suspect that to call something "pleasure," it has to be something that we're aware of enjoying. We need the capacity to say, "Yeah! I like it!"

That raises the whole question of plant intelligence. Just what does it mean to be aware of something? Surely there's a spectrum, and maybe plants and jellyfish overlap with animals on it.